Diode rectifier circuit



rch 17, w36. P. o. FARNHAM 0345M- DIODE REGTIFIER CIRCUIT Filed July 25, 1952 2 Sheets-Sheet l nounou rch 17, 1936. P. o, FARNHAM 2,034,511

DIODE RECTIFIER CIRCUIT Filed July 25, 1952' 2 Sheets-511.66%, 2

Patented Mar. 17, 1936 S TT moms REc'rIFm-R CIRCUIT Delaware Application July 25, 1932', Serial No. 624,567

3. Claims.

This invention relates to diode rectier circuits such as commonly employed for the demodulation of carrier or intermediate frequency signals in radio receivers.

It is generally believed that, when subjected to input voltages above a critical range, a diode rectifier has the inherent property of developing an audio frequency response which is a substantially true copy of the modulation envelope. This accepted belief has arisen from the fact that early work with high voltage diode detectors or rectiiiers was restricted to the use of relatively low percentage modulation and/or that a diode rectifier has a linear audio frequency response for as high as 100% modulation so long as the audio frequency voltage is not impressed upon a load circuit.

It is a fact, however, that the audio frequency voltage which a high voltage diode rectifier impresses upon an audio frequency load, such as an amplifier stage, is not a faithful reproduction of the modulation envelope when the modulation of the impressed signal voltage is relatively high. This audio frequency distortion arises from the fact that although the connecting of an audio frequency load circuit across the output resistor of a diode may not alter the direct current resistance of the output circuit, it

usually reduces its effective audio frequency impedance.

An object of the present invention is to provide diode rectifier circuits which are substantially free from audio distortion, for al1 degrees of modulation between zero and 100%, wheny working into an audio frequency load. An object is to provide diode rectifier circuits for working into an audio frequency load, and in which the audio frequency impedance of the diode output circuit will not be substantially reduced by the addition of the load circuit. More specilically, an object of the invention is to provide-a diode rectifier having an output circuit that includes a direct current resistance and also a reactive impedance, these output elements and their magnitudes being so related that the shunting of an audio frequency load across the output circuit will not reduce the effective impedance of the output circuit to a value substantially lower than its direct current resistance.

These and other objects and advantages of the invention will be apparent from the following specification when taken with the accompanying drawings, in which:

Fig. 1 is a circuit diagram of the essential elements of a diode rectifier and of an associated audio frequency load circuit;

Fig. 2` isa curve sheet showing the relationship between the direct current voltage between the rectifier elements and rectified output current for a plurality of peak carrier voltages, and

Figs. 3 to 8 are circuit diagrams of different embodiments of the invention.

In the drawings, the reference numeral I identil'ies the carrier wave impedance across which the modulated voltage E is developed for application to the diode rectier 2, the audio frequency Voltage Ea resulting from the diode rectication being transferred to a load circuit such as the amplifier tube 3. The output impedance is connected between the points A, B, of the diode circuit and has the form of resistance R shunted by a carrier frequency by-pass condenser C. The audio frequency load is connected across points A, B, and, in the customary circuits, the coupling is effected by connecting the grid of tube 3 to point A through an audio frequency by-pass condenser C1, and connecting the grid to point B through a grid resistance Rg.

The audio frequency voltage Ea and direct current voltage Et developed across the output inipedanee, i. e., between points A, B, for various values of peak carrier voltages E may be determined from a family of voltage-current curves such as sho-wn in Fig. 2. 'I'he numbers adjacent the several curves indicate the values of the peak carrier voltages and, for any given peak voltage, the operating point P of the rectifier is the intersection of the particular curve with a line F drawn through the origin and having a slope determined by the direct current resistance R.

If the audio frequency impedance between points A, B, is equal to the direct current resistance R, operation takes place along line F and, even for modulation, the audio response Ea will be a substantially true copy o-f the modulation envelope.

When the load circuit is added, as shown in Fig. 1, the audio frequency impedance of the diode output circuit is reduced by the shunt load circuit and operation takes place along a line such as G which is drawn through the operating point P with a slope equal to the audio impedance existing between A and B exclusive of tube 2. It is to be noted that this operating line G cuts the axis at a point X which is remote from the origin. The significance of this fact is that the audio frequency response may be cut off over a portion of the modulation cycle since the audio current output falls to zero at the point X. Only the peaks of a highly modulated wave will be reproduced and audio distortion therefore results.

As shown in Fig. 3, the output impedance between points A, B, may include, in series, with the resistance R, an audio frequency choke L of such magnitude that, over the useful audio range, w L R. As in the usual circuits, a carrier frequency by-pass condenser C is shunted across points A, B, and an audio frequency by-pass condenser C2 is shunted across the resistance R. The audio frequency voltage E8. is developed mainly across the choke L, and, therefore, the audio load circuit, CiRg, may be placed in shunt with L without making the effective audio irnpedance between terminals A, B less than the direct current resistance of R over the useful range of received audio frequencies. The path of operation will then be of sufficient slope, correspending to line F or greater, to introduce no audio distortion at modulations as great as A somewhat simplified circuit is shown in Fig. 4, in which the audio load or amplifier tube 3, is connected across only the choke L that is in series with resistance R.

The resistance is by-passed for audio frequency by condenser C2 and, if necessary, a carrier frequency by-pass is provided around choke L by a condenser C. Since the direct current drop across choke L is negligible in comparison with that across resistance R, it is not necessary to include a blocking condenser in the lead to the grid of amplifier 3.

As shown in Fig. 5, the output circuit of the diode includes a resistive load R and an audio choke L between the terminal points A, B, the resistance being shunted by an audio by-pass condenser C2 and a carrier frequency by-pass condenser C being connected between points A and B. A resistance R2 is shunted across the choke L through a blocking condenser C1, and the grid of the amplifier 3 is connected to an acljustable tap 4 on resistance R2, thus permitting a control of the audio output level.

A variation of the latter circuit is shown in Fig. 6, in which the resistance R is not by-passed for audio frequencies, and the load circuit, consisting of the blocking condenser C1 and resistance R2, is connected between points A and B. Exclusive of the carrier frequency by-pass condenser C, and according to a well-known relation, the audio impedance of the diode output circuit will be independent of frequency when and will be equal to R.

Another method of eliminating or reducing the distortion arising when an amplifier input circuit is connected across the diode output circircuit is illustrated in Figs. 7 and 8. Direct coupling is employed and the grid and cathode terminals of the amplifier stage are connected directly to the diode output terminal points A and B. The audio frequency impedance between the grid and cathode of the amplifier is comparatively high and the effective direct current resistance and audio impedance of the diode output circuit are approximately equal. With the customary arrangement of the output impedance connected between ground and the cathode of the diode, as shown in Fig. '7, the flow of current in the resistance R gives point A a positive direct current potential, and with the connections to the diode reversed, as shown in Fig. 8, the potential of point A is more negative than that of the grounded point B. Since these potentials are applied tothe grid of the amplifier 3, the bias on the amplifier will vary as the carrier level changes but this effect may be compensated, in part at least, by direct current degeneration in the amplifier circuits, for example by the use of a cathode resistor R3.

From the several. described embodiments of the invention, it will be apparent that there is a considerable latitude in the design of circuits embodying the invention. Although a substantially complete elimination of audio distortion may be desirable or necessary in some instances, it is to be understood that the invention is not restricted to circuits which effect either an exact or an approximately exact elimination of all distortion.

I claim:

1. In a device of the class described, the combination of a diode rectifier having a cathode and anode, an input circuit including a condenser connecting said cathode and anode, the series connection of a resistor and an inductance shunted across the opposite side of said condenser, an amplifier tube having a cathode, a control grid and a plate, a condenser connecting said control grid to said diode cathode and a connection between said amplifier cathode and the low potential end of said inductance.

2. In a device of the class described, the combination of a diode rectifier having a cathode and anode, an input circuit including at least one condenser connecting said cathode and anode, the series connection of a resistor and an inductance shunted across said condenser, the reactance of said inductance being at least as large as the direct current resistance of said resistor, an amplifier tube having a cathode, a control grid and a plate, a circuit having a capacitive reactance connecting said control grid to said diode cathode and means for maintaining the low potential end of said inductance and said amplifier cathode at substantially the same direct current potential.

3. The combination defined in the preceding claim in which a condenser having a low impedance to currents of audio frequency is connected in shunt across said resistor.

PAUL O. FARNHAM. 

